>We really >need someone from the heavy electrical industry to comment. I'll bite, I worked in the heavy electricial industry for years. Lightning means overtime. If lightning hits the ground, around one time out of three it goes to ground through a high tension power line. If it hits a power line, it knocks out a bunch of high tension fuses. So you spend hours driving around replacing high tension fuses. Easy work and lots of it. Most 11kv/400v transformers have these spark gap arc horns to divert lightning. The lightning jumps across the air gap and goes to ground rather than going through the transformer. It's reasonably rare that lightning takes a transformer out, although the HT fuses come before the spark gap, which is why they blow. So while you definitely get spikes on the low tension side, they tend to be of very short duration, though the voltage can be up to six or eight hundred volts. Now, if you get a lightning strike on a low tension line, then kiss your appliances goodbye. I've seen it. Everything in the house that is plugged in gets shagged. And your insurance company says "Act of God, tough luck mate." (Always a good idea to check to see if you're covered for lightning strike) High tension lines also have the occasional lightning arrester, which when blown apart seem to be made up of lots of disks stacked up into a cylinder. Lightning does raise the ground potential at the instant of strike, and is something to be aware of. There are cases of cows being electrocuted because lightning struck nearby and the difference in potential between the ground at their front legs and the ground at their back legs was enough to electrocute them. So we have this situation where you string a cable from one house to another, is it safe or not? All right. If the two houses are on different supply transformers then don't do it. The reason is this: Your cable can carry current caused by differences in ground potential between the two sites. Each transformer has it's own earth mat, but the only electrical connection between the two transformers is the ground itself. Your cat5 can be a better conductor than the ground. Under normal conditions you may get away with it, under fault conditions your cable may heat up until something melts. OK, two houses on the same transformer are already connected together - by the neutral conductor. So stringing a cable between them is a bit safer. You are paralleling a big fat wire with your little thin wire. However, you can run into trouble like this: In one of the houses the hot water cylinder gets replaced. The plumber disconnects the main earth conducter from the copper pipes and doesn't reconnect it, leaving the house with no bond to earth. A tree then falls through the overhead line, taking out the neutral, but not the phase. So electricity can get into the house, but has no return path. If you had strung a bit of coax between this house and the next for your 10base2 network, then you're in trouble, because the shield on the coax will try to carry the whole return current for the house. (Actually, you're in trouble anyway, because every metal appliance in the house would bite you if you touched it) Now your 10baseT network should actually put up with this problem because of the 2kv isolation. But if it breaks down at one end then you get the full voltage at the other end. Which the NIC at the other end may put up with, but it is not good for you if you unplug the cable and touch the end. Under normal conditions, stringing cat5 a short distance between 2 houses on the same distribution transformer would be fine. Under abnormal conditions expect trouble. Plan for the abnormal conditions. Wayne ------------------------------------------------------------------- To remove yourself from this list, email nzlug-request@linux.net.nz with "unsubscribe" in the body of the message.
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